One of the critical problems with transparent enclosures for solar power harvesters is the presence of water vapor in the enclosed volume. Upon cooling at night the water vapor condenses first on the air-cooled transparent window, forming droplets and “fogging” the inner window surface The next day the solar collector is covered with a diffusing sheet of water droplets and the collector forms no image. In cold climates this further freezes to form frost on the window: the rising sun illuminates a diffusing, highly reflective sheet over the solar collector, and the frost remains substantially unheated, perhaps throughout the day unless the air warms enough to “burn it off”.
Thus the condensation of water on the interior of the transparent cover of the collector effectively shuts down power harvesting, and even shuts down the ability of the window to clear itself in cold climates: the windows may remain frosted throughout the day with zero net power generated by the solar power harvester system.
This is a common problem with solar collectors. Typically the problem is ignored, and the user simply waits for the heat of the sun to melt the frost and to evaporate the condensed water. At minimum a significant portion of the power-generating capability is lost at the start of the day; in colder climates the frosted layer on the air-chilled window may not abate during the day, and thus the condensed water vapor prevents the system from operating at all.
U.S. Pat. No. 4,803,972 teaches the use of a hermetically sealed chamber with the chamber allowed to expand and contract according to the pressure. This has the problem of scale: the chamber pressure between a freezing night and a solar-heated 50° C. the next noon corresponds to an 18% change in chamber volume: a generally impractical movement of the face of the transparent cover.
A method for defogging the lenses of goggles is described in U.S. Pat. No. 4,414,693, which teaches the use of desiccator elements built into the goggles housing; such a desiccator element becomes useless when it is saturated with water.
U.S. Pat. No. 7,178,355 and its many predecessors teach the use of a desiccant wheel as a method for continuously recharging a desiccator element by cyclically passing the saturated desiccant element through a heated zone to drive off the adsorbed water, then through an exposure zone to adsorb water.
Solar power harvesters heretofore known suffer from a number of disadvantages:
(a) They suffer condensation of water vapor present in the interior of the transparent housing, as the air cools after sundown.
(b) The presence of water condensed on the interior of the transparent window causes sunlight to be diffused and scattered back away from the absorptive interior, and prevents focusing of the sunlight into a focal spot.
(c) The window becomes rapidly soiled: condensed water acts as a sink for airborne dust, “pumping” the airborne dust onto the inner window surface; the window accumulates a sun-blocking layer of dirt which cannot be removed except by system disassembly.
(d) After nights when the ambient temperature drops below freezing, frost on the window interior is highly reflective of morning sunlight, and little energy is transmitted to the chamber interior; that transmitted light is scattered rather than collimated for the use of concentrating optics; and the frost will not dissipate until the ambient air heats up enough to melt the frost and then even more to evaporate the resultant liquid condensation.
(e) Thus condensed water vapor is very costly to the average annual yield of useful power, and practicable methods have not been previously taught for excluding water vapor from the interior of the solar harvester housing.